In Greek mythology, Achelous was the patron deity of the river by the same name, and the chief god of all river deities (every river had a specific god assigned to it). His name translates as "he who washes away care". He was the eldest child of Oceanus and Tethys. He was portrayed in two different fashions. Most frequently, he was a bull with the torso of a man. Occasionally, however, he was shown as a gray-haired old man with horns. He was also considered a storm-god.

Achelous was killed by Heracles in a dispute over a princess named Deianira. After his death, Heracles gave one of his horns to a group of nymphs, who transformed it into the cornucopia.

He was sometimes the father of the Sirens by Terpsichore.

The river Achelous was said to be formed by the tears of Niobe, who fled to Mt. Sipylon after the deaths of her husband and children.

The mouth of the Achelous river was the spot where Alcmaeon finally found peace from the Erinyes. Achelous offered him Callirhoe, his daughter, in marriage if Alcmaeon would retrieve the clothing and jewelry his mother, Eriphyle, had been wearing when she sent her husband, Amphiaraus to his death. Alcmaeon had to retrieve the clothes from King Phegeus, who sent his sons to kill Alcmaeon.

Ovid VIII, 547; IX, 1